<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>计数器</title>
</head>
<body>
  
  <div id="root"></div>

  <script src="../lib/react.js"></script>
  <script src="../lib/react-dom.js"></script>
  <script src="../lib/babel.js"></script>

  <script type="text/babel">
    // 1.定义App根组件
    class App extends React.Component {
      constructor() {
        super()
        this.state = {
          message: "Hello World",

          isReady: false,

          friend: undefined
        }
      }

      render() {
        const { isReady, friend } = this.state

        // 1.条件判断方式一: 使用if进行条件判断
        let showElement = null
        if (isReady) {
          showElement = <h2>准备开始比赛吧</h2>
        } else {
          showElement = <h1>请提前做好准备!</h1>
        }

        return (
          <div>
            {/* 1.方式一: 根据条件给变量赋值不同的内容 */}
            <div>{showElement}</div>

            {/* 2.方式二: 三元运算符 */}
            <div>{ isReady ? <button>开始战斗!</button>: <h3>赶紧准备</h3> }</div>

            {/* 3.方式三: &&逻辑与运算 */}
            {/* 场景: 当某一个值, 有可能为undefined时, 使用&&进行条件判断 */}
            <div>{ friend && <div>{friend.name + " " + friend.desc}</div> }</div>
          </div>
        )
      }
    }

    // 2.创建root并且渲染App组件
    const root = ReactDOM.createRoot(document.querySelector("#root"))
    root.render(<App/>)
  </script>

</body>
</html>